// Ami Bar
// amibar@gmail.com

using System;
using System.Collections;
using System.Diagnostics;

namespace Amib.Threading.Internal
{

    #region PriorityQueue class

    /// <summary>
    /// PriorityQueue class
    /// This class is not thread safe because we use external lock
    /// </summary>
    public sealed class PriorityQueue : IEnumerable
    {
        #region Private members

        /// <summary>
        /// The number of queues, there is one for each type of priority
        /// </summary>
        private const int _queuesCount = WorkItemPriority.Highest - WorkItemPriority.Lowest + 1;

        /// <summary>
        /// Work items queues. There is one for each type of priority
        /// </summary>
        private readonly Queue[] _queues = new Queue[_queuesCount];

        /// <summary>
        /// Use with IEnumerable interface
        /// </summary>
        private int _version;

        /// <summary>
        /// The total number of work items within the queues 
        /// </summary>
        private int _workItemsCount;

        #endregion

        #region Contructor

        public PriorityQueue()
        {
            for (int i = 0; i < _queues.Length; ++i)
            {
                _queues[i] = new Queue();
            }
        }

        #endregion

        #region Methods

        /// <summary>
        /// The number of work items 
        /// </summary>
        public int Count
        {
            get { return _workItemsCount; }
        }

        /// <summary>
        /// Enqueue a work item.
        /// </summary>
        /// <param name="workItem">A work item</param>
        public void Enqueue(IHasWorkItemPriority workItem)
        {
            Debug.Assert(null != workItem);

            int queueIndex = _queuesCount - (int) workItem.WorkItemPriority - 1;
            Debug.Assert(queueIndex >= 0);
            Debug.Assert(queueIndex < _queuesCount);

            _queues[queueIndex].Enqueue(workItem);
            ++_workItemsCount;
            ++_version;
        }

        /// <summary>
        /// Dequeque a work item.
        /// </summary>
        /// <returns>Returns the next work item</returns>
        public IHasWorkItemPriority Dequeue()
        {
            IHasWorkItemPriority workItem = null;

            if (_workItemsCount > 0)
            {
                int queueIndex = GetNextNonEmptyQueue(-1);
                Debug.Assert(queueIndex >= 0);
                workItem = _queues[queueIndex].Dequeue() as IHasWorkItemPriority;
                Debug.Assert(null != workItem);
                --_workItemsCount;
                ++_version;
            }

            return workItem;
        }

        /// <summary>
        /// Find the next non empty queue starting at queue queueIndex+1
        /// </summary>
        /// <param name="queueIndex">The index-1 to start from</param>
        /// <returns>
        /// The index of the next non empty queue or -1 if all the queues are empty
        /// </returns>
        private int GetNextNonEmptyQueue(int queueIndex)
        {
            for (int i = queueIndex + 1; i < _queuesCount; ++i)
            {
                if (_queues[i].Count > 0)
                {
                    return i;
                }
            }
            return -1;
        }

        /// <summary>
        /// Clear all the work items 
        /// </summary>
        public void Clear()
        {
            if (_workItemsCount > 0)
            {
                foreach (Queue queue in _queues)
                {
                    queue.Clear();
                }
                _workItemsCount = 0;
                ++_version;
            }
        }

        #endregion

        #region IEnumerable Members

        /// <summary>
        /// Returns an enumerator to iterate over the work items
        /// </summary>
        /// <returns>Returns an enumerator</returns>
        public IEnumerator GetEnumerator()
        {
            return new PriorityQueueEnumerator(this);
        }

        #endregion

        #region PriorityQueueEnumerator

        /// <summary>
        /// The class the implements the enumerator
        /// </summary>
        private class PriorityQueueEnumerator : IEnumerator
        {
            private readonly PriorityQueue _priorityQueue;
            private IEnumerator _enumerator;
            private int _queueIndex;
            private int _version;

            public PriorityQueueEnumerator(PriorityQueue priorityQueue)
            {
                _priorityQueue = priorityQueue;
                _version = _priorityQueue._version;
                _queueIndex = _priorityQueue.GetNextNonEmptyQueue(-1);
                if (_queueIndex >= 0)
                {
                    _enumerator = _priorityQueue._queues[_queueIndex].GetEnumerator();
                }
                else
                {
                    _enumerator = null;
                }
            }

            #region IEnumerator Members

            public void Reset()
            {
                _version = _priorityQueue._version;
                _queueIndex = _priorityQueue.GetNextNonEmptyQueue(-1);
                if (_queueIndex >= 0)
                {
                    _enumerator = _priorityQueue._queues[_queueIndex].GetEnumerator();
                }
                else
                {
                    _enumerator = null;
                }
            }

            public object Current
            {
                get
                {
                    Debug.Assert(null != _enumerator);
                    return _enumerator.Current;
                }
            }

            public bool MoveNext()
            {
                if (null == _enumerator)
                {
                    return false;
                }

                if (_version != _priorityQueue._version)
                {
                    throw new InvalidOperationException("The collection has been modified");
                }
                if (!_enumerator.MoveNext())
                {
                    _queueIndex = _priorityQueue.GetNextNonEmptyQueue(_queueIndex);
                    if (-1 == _queueIndex)
                    {
                        return false;
                    }
                    _enumerator = _priorityQueue._queues[_queueIndex].GetEnumerator();
                    _enumerator.MoveNext();
                    return true;
                }
                return true;
            }

            #endregion
        }

        #endregion
    }

    #endregion
}